1. Field of the Invention
The present invention relates to an electric motor, and more specifically relates to an electric motor having a structure in which three-layer windings are arranged in slots of a stator.
2. Description of Related Art
To reduce torque ripple, optimizations have been conventionally performed on the forms of rotor cores, the forms of stator cores, the skews of the rotor cores, the skews of the stator cores, and the like.
However, such modifications to electric motors require that the electric motors have more complicated structures, causing an increase in the number of process steps in manufacture.
Also, in the electric motors, the magnitude of the torque ripple is affected by the number of poles, the number of slots, and the winding arrangement of windings.
As a combination of the pole number and the slot number that serves to reduce the torque ripple, there is a fractional slot electric motor in which the division of the slot number by the pole number is a fraction in lowest terms (for example, Japanese Patent Application Laid-Open Nos. 2004-23950 and S62-250851).
In the fractional slot electric motor, determining the pole number and the slot number such that the pole number and the slot number have a high least common multiple facilitates reducing the torque ripple. However, of the harmonic components of an induced voltage of the electric motor, harmonic components of relatively low orders do not completely disappear and cause torque ripple.
There is also another problem in that the reduction in the torque ripple brings about a reduction in a winding factor of first order, thus resulting in a reduction in torque.